Conventionally, a light irradiation device that irradiates a wire with ultraviolet light to harden a resin coating on the outer surface of the wire and/or reform the surface of the wire is known. In a process for manufacturing an optical fiber, for example, the resin coating is applied on the outer surface of an optical fiber to form a protective film on the outer surface of the optical fiber.
For example, as shown in FIG. 8 of the accompanying drawings, Japanese Patent No. 5851837 (Patent Literature Document 1) discloses a light irradiation device that directs light, which is emitted from a discharge lamp, around the wire by a reflection mirror. In this light irradiation device, a discharge lamp 101 is placed at a first focal point of a light converging (condensing) reflection mirror 102, and a wire 104 that extends in a protective tube 103 arranged at a position facing the discharge lamp is placed at a second focal point. With such configuration, the ultraviolet light from the discharge lamp 101 is reflected and converged by the light converging reflection mirror 102 such that the wire 104 extending in the protective tube 103 is irradiated with the ultraviolet light to harden the wire 104 with the light.
In this prior art technology, there is a problem that the coating material applied on the wire flies during conveyance, and the coating material stains and/or contaminates the protective layer that is arranged around the wire. If the protective tube is stained and/or contaminated, a problem arises, i.e., the light to be directed to the wire is attenuated, and it becomes difficult to efficiently irradiate the wire with the light for the hardening. To address this problem, it is necessary to replace the protective tube with a new protective tube at regular intervals. However, if the reflection mirror is scratched and/or contaminated during the protective tube replacement work, this would deteriorate the reflectance.
When manufacturing the reflection mirror, a metal block material undergoes a mirror-finishing process. The above-mentioned replacement work of the protective tube inevitably damages the reflection mirror.
Japanese Patent No. 2892545 (Patent Literature Document 2) teaches that a reflection mirror is made integrally with a reflection mirror holding body, and the reflection mirror is removed together with the reflection mirror holding body. This configuration, however, entails a problem that the thickness of the reflection mirror becomes small, and the reflection mirror deforms due to thermal expansion. If the curvature of the reflection mirror changes due to the thermal expansion, a problem arises, i.e., the focal point shifts from a designed point, and this makes it difficult to efficiently harden the wire.